6.3. Hooks and Operations#

6.3.1. Example using dataflow hooks#

We will use a very simple example to show hooks usage. Let us start with the following schema.

class Person(EntityType):
    age = Int(required=True)

We would like to add a range constraint over a person’s age. Let’s write an hook (supposing yams can not handle this natively, which is wrong). It shall be placed into mycube/hooks.py. If this file were to grow too much, we can easily have a mycube/hooks/
 package containing hooks in various modules.

from cubicweb import ValidationError
from cubicweb.predicates import is_instance
from cubicweb.server.hook import Hook

class PersonAgeRange(Hook):
     __regid__ = 'person_age_range'
     __select__ = Hook.__select__ & is_instance('Person')
     events = ('before_add_entity', 'before_update_entity')

     def __call__(self):
         if 'age' in self.entity.cw_edited:
             if 0 <= self.entity.age <= 120:
                return
             msg = self._cw._('age must be between 0 and 120')
             raise ValidationError(self.entity.eid, {'age': msg})

In our example the base __select__ is augmented with an is_instance selector matching the desired entity type.

The events tuple is used to specify that our hook should be called before the entity is added or updated.

Then in the hook’s __call__ method, we:

  • check if the ‘age’ attribute is edited

  • if so, check the value is in the range

  • if not, raise a validation error properly

Now let’s augment our schema with a new Company entity type with some relation to Person (in ‘mycube/schema.py’).

class Company(EntityType):
     name = String(required=True)
     boss = SubjectRelation('Person', cardinality='1*')
     subsidiary_of = SubjectRelation('Company', cardinality='*?')

We would like to constrain the company’s bosses to have a minimum (legal) age. Let’s write an hook for this, which will be fired when the boss relation is established (still supposing we could not specify that kind of thing in the schema).

class CompanyBossLegalAge(Hook):
     __regid__ = 'company_boss_legal_age'
     __select__ = Hook.__select__ & match_rtype('boss')
     events = ('before_add_relation',)

     def __call__(self):
         boss = self._cw.entity_from_eid(self.eidto)
         if boss.age < 18:
             msg = self._cw._('the minimum age for a boss is 18')
             raise ValidationError(self.eidfrom, {'boss': msg})

Note

We use the match_rtype selector to select the proper relation type.

The essential difference with respect to an entity hook is that there is no self.entity, but self.eidfrom and self.eidto hook attributes which represent the subject and object eid of the relation.

Suppose we want to check that there is no cycle by the subsidiary_of relation. This is best achieved in an operation since all relations are likely to be set at commit time.

from cubicweb.server.hook import Hook, DataOperationMixIn, Operation, match_rtype

def check_cycle(session, eid, rtype, role='subject'):
    parents = set([eid])
    parent = session.entity_from_eid(eid)
    while parent.related(rtype, role):
        parent = parent.related(rtype, role)[0]
        if parent.eid in parents:
            msg = session._('detected %s cycle' % rtype)
            raise ValidationError(eid, {rtype: msg})
        parents.add(parent.eid)


class CheckSubsidiaryCycleOp(Operation):

    def precommit_event(self):
        check_cycle(self.session, self.eidto, 'subsidiary_of')


class CheckSubsidiaryCycleHook(Hook):
    __regid__ = 'check_no_subsidiary_cycle'
    __select__ = Hook.__select__ & match_rtype('subsidiary_of')
    events = ('after_add_relation',)

    def __call__(self):
        CheckSubsidiaryCycleOp(self._cw, eidto=self.eidto)

Like in hooks, ValidationError can be raised in operations. Other exceptions are usually programming errors.

In the above example, our hook will instantiate an operation each time the hook is called, i.e. each time the subsidiary_of relation is set. There is an alternative method to schedule an operation from a hook, using the get_instance() class method.

class CheckSubsidiaryCycleHook(Hook):
    __regid__ = 'check_no_subsidiary_cycle'
    events = ('after_add_relation',)
    __select__ = Hook.__select__ & match_rtype('subsidiary_of')

    def __call__(self):
        CheckSubsidiaryCycleOp.get_instance(self._cw).add_data(self.eidto)

class CheckSubsidiaryCycleOp(DataOperationMixIn, Operation):

    def precommit_event(self):
        for eid in self.get_data():
            check_cycle(self.session, eid, self.rtype)

Here, we call add_data() so that we will simply accumulate eids of entities to check at the end in a single CheckSubsidiaryCycleOp operation. Values are stored in a set associated to the ‘check_no_subsidiary_cycle’ transaction data key. The set initialization and operation creation are handled nicely by add_data().

A more realistic example can be found in the advanced tutorial chapter Step 2: security propagation in hooks.

6.3.2. Inter-instance communication#

If your application consists of several instances, you may need some means to communicate between them. Cubicweb provides a publish/subscribe mechanism using ØMQ. In order to use it, use add_subscription() on the repo.app_instances_bus object. The callback will get the message (as a list). A message can be sent by calling publish() on repo.app_instances_bus. The first element of the message is the topic which is used for filtering and dispatching messages.

class FooHook(hook.Hook):
    events = ('server_startup',)
    __regid__ = 'foo_startup'

    def __call__(self):
        def callback(msg):
            self.info('received message: %s', ' '.join(msg))
        self.repo.app_instances_bus.add_subscription('hello', callback)

def do_foo(self):
    actually_do_foo()
    self._cw.repo.app_instances_bus.publish(['hello', 'world'])

The zmq-address-pub configuration variable contains the address used by the instance for sending messages, e.g. tcp://*:1234. The zmq-address-sub variable contains a comma-separated list of addresses to listen on, e.g. tcp://localhost:1234, tcp://192.168.1.1:2345.

6.3.3. Hooks writing tips#

6.3.3.1. Reminder#

You should never use the entity.foo = 42 notation to update an entity. It will not do what you expect (updating the database). Instead, use the cw_set() method or direct access to entity’s cw_edited attribute if you’re writing a hook for ‘before_add_entity’ or ‘before_update_entity’ event.

6.3.3.2. How to choose between a before and an after event ?#

before_* hooks give you access to the old attribute (or relation) values. You can also intercept and update edited values in the case of entity modification before they reach the database.

Else the question is: should I need to do things before or after the actual modification ? If the answer is “it doesn’t matter”, use an ‘after’ event.

6.3.3.3. Validation Errors#

When a hook which is responsible to maintain the consistency of the data model detects an error, it must use a specific exception named ValidationError. Raising anything but a (subclass of) ValidationError is a programming error. Raising it entails aborting the current transaction.

This exception is used to convey enough information up to the user interface. Hence its constructor is different from the default Exception constructor. It accepts, positionally:

  • an entity eid (not the entity itself),

  • a dict whose keys represent attribute (or relation) names and values an end-user facing message (hence properly translated) relating the problem.

raise ValidationError(earth.eid, {'sea_level': self._cw._('too high'),
                                  'temperature': self._cw._('too hot')})

6.3.3.4. Checking for object created/deleted in the current transaction#

In hooks, you can use the added_in_transaction() or deleted_in_transaction() of the session object to check if an eid has been created or deleted during the hook’s transaction.

This is useful to enable or disable some stuff if some entity is being added or deleted.

if self._cw.deleted_in_transaction(self.eidto):
   return

6.3.3.5. Peculiarities of inlined relations#

Relations which are defined in the schema as inlined (see Relation type for details) are inserted in the database at the same time as entity attributes.

This may have some side effect, for instance when creating an entity and setting an inlined relation in the same rql query, then at before_add_relation time, the relation will already exist in the database (it is otherwise not the case).